With an annual production of >10 million t (Mt), China is the largest producer of cultivated shellfish and seaweeds in the world. Through mariculture of shellfish and seaweeds, it is estimated that 3.79 ± 0.37 Mt C yr -1 are being taken up, and 1.20 ± 0.11 Mt C yr -1 are being removed from the coastal ecosystem by harvesting (means ± SD). These estimates are based on carbon content data of both shellfish and seaweeds and annual production data from 1999 to 2008. The result illustrates that cultivated shellfish and seaweeds can indirectly and directly take up a significant volume of coastal ocean carbon -shellfish accomplish this by removal of phytoplankton and particulate organic matter through filter feeding, and seaweeds through photosynthesis. Thus, cultivation of seaweeds and shellfish plays an important role in carbon fixation, and therefore contributes to improving the capacity of coastal ecosystems to absorb atmospheric CO 2 . Because the relationship between mariculture and the carbon cycle of the coastal ecosystem is complicated and the interaction between the 2 processes is significant, such studies should be continued and given high priority.

Integrated multi-trophic aquaculture (IMTA) involves the farming of species from different trophic positions or nutritional levels in the same system. In China, IMTA has been practiced for many decades, with dozens of species farmed in close proximity to each other at the scale of whole coastal bays. Articles in this Theme Section present results from the MoST-China Project on 'Sustainability of Marine Ecosystem Production under Multi-stressors and Adaptive Management ' (2011−2015). This project sought to understand the interactions between biogeochemical cycles and ecosystem function in the IMTA system of Sanggou Bay, China, which produces a total of > 240 000 t of seafood each year from > 30 species in approximately 100 km 2 of production space. Results include measurements of carbon, nitrogen flow and trophic relationships among cultured species; impacts of IMTA on benthic nutrient fluxes, reduced inorganic sulfur in sediments, distribution of dissolved inorganic selenium, and nutrient cycling; distribution and seasonal variation of picoplankton; and a model for kelp growth. Combined, the articles enable a complex understanding of the dynamics between IMTA and the environment in one of the most important coastal aquaculture production systems in the world.

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Picoplankton abundance and biomass in Sanggou Bay, China, were investigated in 4 successive seasons (April, August and October 2011, January 2012). Different distribution patterns of picoplankton abundance and biomass were observed according to season and culture areas (bivalves or macroalgae). Synechococcus, picoeukaryotes and heterotrophic prokaryotes exhibited higher abundance and biomass in warm seasons (summer and autumn) than in cold seasons (spring and winter). Over all 4 seasons, picoplankton abundance was higher in the bivalve culture area than in the macroalgae culture area. Among picoplankton, picoeukaryotes contributed most to the carbon standing stock in summer and autumn. In spring and winter, the heterotrophic component biomass exceeded that of the autotrophic picoplankton. Picoeukaryotes were an important contributor (21−27%) to total phytoplankton carbon biomass in spring to autumn. In spring, heterotrophic prokaryote biomass accounted for more than 56% of total phytoplankton biomass, and even exceeded phytoplankton biomass at some stations. As revealed by multiple stepwise regression analysis, physicochemical factors and protist grazing were the most important variables that controlled picoplankton distribution and variation. The reduction in grazing pressure, as well as phosphorus release by bivalves, is likely to explain the higher abundance of picoplankton in the bivalve culture area of Sanggou Bay.

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